Hitherto, FM-CW radars using a millimeter wave have been developed, as on-vehicle radars, for example. The FM-CW radar detects an object by transmitting and receiving a radio wave including a frequency-modulated (FM) continuous wave (CW). That is to say, the FM-CW radar transmits a transmission signal including an ascending-modulation section and a descending-modulation section that are observed in an alternating manner, where a frequency gradually ascends in the ascending-modulation section and gradually descends in the descending-modulation section, receives a reception signal including a reflection signal transmitted from the object, and obtains the relative distance and relative speed of the object based on the frequency spectrums of beat signals denoting the frequency difference between the transmission signal and the reception signal. Further, the FM-CW radar performs the above-described operations for a single beam directed to a predetermined azimuth and changes the beam azimuth in sequence, thereby calculating the azimuths of objects distributed over a predetermined azimuth-angle range.
In the case where a single object exists, a single projection portion is observed in each of the frequency spectrums of beat signals corresponding to a reflection wave transmitted from the object in the ascending-modulation section and the descending-modulation section. Therefore, the relative distance and relative speed of the object can be obtained based on the peak frequencies of the projection portions, where the peak frequencies include the beat-signal frequency in the ascending-modulation section (hereinafter referred to as an “upbeat frequency”) and the beat-signal frequency in the descending-modulation section (hereinafter referred to as a “downbeat frequency”).
However, where a plurality of objects exists in a predetermined detection range, a plurality of projection portions is observed in the frequency spectrums, for one and the same beam, in the ascending-modulation section and the descending-modulation section. Therefore, an error may occur in combinations of a plurality of upbeat frequencies and a plurality of downbeat frequencies (hereinafter referred to as “pairing”).
Further, where the FM-CW radar is used, as the on-vehicle radar, it is crucial to determine whether an object is a vehicle or a stationary object.
Subsequently, Document No. 1: Japanese Unexamined Patent Application Publication No. 7-98375 discloses a radar that determines a predetermined object to be a stationary object, where the relative speed of the object is the same as the vehicle speed. Document No. 2: Japanese Unexamined Patent Application Publication No. 5-232214 discloses a radar that determines a predetermined object to be a stationary object, where the relative speed of the object is the same as the vehicle speed and where the spectrums of beat signals of the object spread out. Document No. 3: Japanese Unexamined Patent Application Publication No. 11-211811 discloses a radar that determines a predetermined object to be a continuous road-side object, where the frequency spectrums of the beat signals have a peak with a predetermined density or more. Further, Document No. 4: Japanese Unexamined Patent Application Publication No. 2000-147103 discloses a radar that infers data on a stationary object from the stationary-object-location data in the past.
However, according to the research of inventors of the present invention, the above-described known radars have the following problems.
Where an error occurs during pairing, the radars disclosed in Documents No. 1 and No. 2 cannot detect a stationary object and erroneously detects a moving object at a predetermined distance in the direction of the stationary object.
The radar disclosed in Document No. 3 cannot detect a predetermined object, as a road-side object, where the object has a small width in the azimuth direction, as in the case of a road sign, a column of some kind, and so forth.
In the case of a continuous road-side object or the like, the intensity of a reflection signal transmitted from the same position (azimuth) is not high at all times. Therefore, it is not easy for the radar disclosed in Document No. 4 to extract the continuous road-side object based on historical data.
Subsequently, an object of the present invention is to provide a radar for solving the above-described problems, so as to detect a stationary object with ease and stability, and reduce mispairing.